In the recent years, optical fibers have evolved rapidly, which has allowed the use of the fibers outside the traditional telecommunications domain. The progress has been most dynamic in manufacturing, defense, and aerospace industry where fiber lasers are gradually replacing the standard solid state or gas lasers by offering the same or better performance at lower cost and new applications.
The transition from telecom grade fibers to fibers for laser applications involved many design and fabrication changes such as new dopants, new geometries, new structures, and new coating materials. In most of the fiber laser applications, there are two very important fiber design parameters that define the fiber quality. The first one is a maximum optical power handling capability and the second one is a beam quality.
Since silica has been established as an optimum raw material for optical fibers, when the overall characteristics such as availability, cost, and processing knowledge, in order to increase a maximum available power in a fiber laser, one should increase the cross-sectional dimension of the waveguide (the core diameter of the fiber). This reduces the power density and thus a higher total power may be generated or carried by the fiber. However, if the core diameter exceeds some limit the beam quality will suffer due to the excitation of the high order modes. Consequently, there is a fundamental tradeoff between the maximum power and the beam quality for traditional step index fibers.
Several techniques have been developed, which affect the beam quality, for increasing the power handling capability. The standard method is to reduce a refractive index difference between the core and the cladding of the fiber. This reduces the number of higher order modes supported and thus increases the beam quality. However, this method is limited by the fact that lowering the refractive index difference increases the macrobending losses of the fiber, so, its applicability is limited.
Another technique uses bending losses for mode discriminations. Since the bending induced losses are mode dependent, a carefully designed bending radius will yield high losses for high order modes and low losses for a fundamental mode. This method is also limited by the fact that a fiber lifetime decreases as the bending radius decreases.
One way to discriminate the high order modes in a fiber is a technique, which uses the control of refractive index and/or active doping profiles. By careful profile design the high order modes will experience different gain in the fiber. However, this technique requires an advanced manufacturing technology.
It is also possible to use the photonic crystal fiber technology to create waveguides that are fundamentally singlemode for very large core diameters. Usually these fibers require advanced manufacturing technology and have usability issues such as difficult splicing and fragility.
In many practical applications, these aforesaid methods are combined in order to provide the best beam quality for the highest power available.